This invention relates to a heat exchanger for oil used in machinery. More particularly but not exclusively, it relates to a heat exchanger for the oil in the main spindle system of a machine tool.
Conventionally, a heat exchanger for cooling the oil for the main spindle system of a machine tool employs a refrigerator which is turned on and off in response to the temperature of the oil. FIG. 1 is a schematic diagram of one such conventional heat exchanger which is disclosed in "Machine Technology" (Vol. 29, No. 6 (1981), page 101, published by The Daily Industrial News Co., in Japanese). This conventional heat exchanger has an oil tank 2 into which hot oil 1 from the main spindle system of an unillustrated machine tool is discharged. The hot oil 1 is drawn out of the oil tank 2 by a pump 3, and is transported through piping 4 into the bottom of a cooling tank 5. The cooling tank 5 comprises an outer tank 5a and an inner tank 5b which has a cooling pipe 6 wound around it. The opposite ends of the cooling pipe 6 are connected to a refrigerator 7 by piping 8 and 9, respectively. Hot oil 1 which is introduced into the bottom of the outer tank 5a flows upwards through the space between the outer tank 5a and the inner tank 5b, and then overflows into the inner tank 5b. Inside the inner tank 5b, it is cooled by a refrigerant which flows from the refrigerator 7 into the cooling pipe 6 via piping 9 at a low temperature and then returns to the refrigerator 7 via piping 8 at a higher temperature, as shown by the arrows in the figure. Cooled oil 11 which was cooled by the cooling pipe 6 is then drawn from near the bottom of the inner tank 5b through oil supply piping 10 and is returned to the main spindle system of the unillustrated machine tool. A thermostat 12 is disposed in the supply piping 10 where it can detect the temperature of the cooled oil 11. An output signal from the thermostat 12 is input to an unillustrated controller, which controls the operation of the refrigerator 7.
When the temperature of the cooled oil 11 passing through the supply piping 10 is above a certain temperature, the thermostat 12 turns the refrigerator 7 on, and the refrigerator 7 delivers refrigerant to the cooling pipe 6 at a constant rate. When the temperature of the oil 11 falls below a certain temperature, the thermostat 12 turns the refrigerator 7 off to prevent overcooling of the oil 11, and the refrigerator 7 remains off until the oil temperature again rises above a certain level.
Therefore, unless the rate at which the refrigerator 7 performs cooling exactly matches the rate at which the unillustrated machine tool generates heat, the temperature of the cooled oil 11 will fluctuate periodically as the refrigerator 7 is turned on and off. Since the refrigerator 7 has a fixed cooling capacity which is chosen to handle the maximum rate at which the machine tool heats the oil 1, temperature fluctuations will be particularly pronounced due to the repeated turning on and off of the refrigerator 7 that occurs when the machine tool is generating only a small amount of heat. The fluctuations in the oil temperature cause fluctuations in the accuracy of machining, which makes this type of heat exchanger unsuitable when high machining accuracy is required.